Comparisons were made of digestibility coefficients for nutrients in a dry pellet diet fed to three sizes (18, 207, 586 g) of rainbow trout, Salmo gairdneri, at three water temperatures (7, 11, 15 C), and for three meal sizes (0.4, 0.8, 1.6 of fish weight) fed to one fish size (194 g) at one water temperature (11 C). No significant differences were apparent between body sizes at temperatures of 11 and 15 C. However, digestibility coefficients for all nutrients were significantly lower for 18-g trout acclimated to 7 C than 207- or 586-g trout at 7 C or 18-g fish acclimated to 11 and 15 C. The grand average coefficients for protein, lipid, and energy, excluding small fish at 7 C, were 93, 89, and 74 whereas values for the small fish were 20, 9, and 28 lower, respectively. No difference was apparent in the digestibility of protein or lipid at rations of 0.4, 0.8, and 1.6 of the fish body weight. Future digestibility studies with rainbow trout may expect no effect of water temperature and body size within the ranges in this study except for small fish (18 g) at the lowest temperature (7 C). Meal sizes should approximate those fed under normal culture conditions.
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